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The last massive earthquake that shook the south coast of British
Columbia took place on Jan. 26, 1700, say researchers who have been able
to use sediment samples taken from the sea floor off the coast of
Vancouver Island to reveal the Pacific coast's seismic history.
In a study published Wednesday, the team said the region that
stretches from the northern tip of Vancouver Island down the coast to
northern California has experienced 22 major earthquakes over the last
11,000 years, and is due for another.
Audrey Dallimore, of the School of Environment and Sustainability at
Royal Roads University and the author of the study, said the research
showed earthquakes occur every 500 to 1,000 years.
The last one took place 313 years ago.
"What that means is we're due for another subduction zone earthquake either tomorrow — or 700 years from now," she said.
"[It] may happen within our lifetimes and will certainly happen at
some time over the life of our communities and our infrastructures."
Researchers extracted a sediment core from the sea floor of Effingham
Inlet, in Barclay Sound on the west coast of Vancouver Island, and used
radiocarbon dating to determine when large or so-called megathrust
earthquakes occurred on what is known as the Cascadia subduction zone.
"The sediments preserved on the bottom of Effingham Inlet resemble
the rings of a tree," Dallimore said, explaining that each year is
represented by a thin layer of sediment.
"These layers have given us a story of what happened in Effingham
Inlet year by year back all the way to the end of the last glaciations
about 11,000 years ago."

No megathrust quake in Canada's written history

By
radiocarbon dating interruptions in the sediment, researchers
determined large earthquakes also took place about 1,200 and 4,000 years
ago.
The first seismograph was installed in Victoria in 1898, so written
records of B.C. seismic activity goes back only a little more than a
century but Japanese written records confirm that a tsunami occurred
from a magnitude-8 or 9 quake along the North American coast about 9
p.m. on Jan 26, 1700.
A megathrust earthquake occurs when a piece of the earth's crust is forced underneath another plate.
The quakes are of magnitude 9 or greater, and both the 2004 Indian
Ocean earthquake that caused the tsunami in Indonesia and the 2011
earthquake and tsunami in Japan were megathrust events.
There has never been a megathrust earthquake along the west coast in
the written history of Canada, but the study confirmed First Nations
oral histories and found that megathrust earthquakes occur about every
500 years in the region, although they can stretch out for up to 1,000
years.
A magnitude 7.7 earthquake that occurred off the west coast of Haida
Gwaii last October was the second-largest ever recorded in Canada but it
was not a megathrust quake.
B.C. forms part of the North American portion of what is called the
Pacific "Ring of Fire," a 40,000-kilometre horseshoe of ocean trenches
and volcanoes where 90 per cent of the world's earthquakes take place.

According to Natural Resources
Canada, the Geological Survey of Canada records more than 1,000
earthquakes in western Canada each year. More than 100 magnitude 5 or
greater earthquakes have been recorded in the ocean west of Vancouver
Island in the past 70 years. READ MORE

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